1. Field of the Invention
This invention relates to an input-output unit for communicating between industrial control equipment and field devices, and more particularly, to improvements therein whereby such unit is easier to assemble, simplifies instrumentation operations, and enables the mounting thereof with increased density.
2. Description of the Prior Art
An input-output unit is housed in a box type body, such as disclosed in Japan Laid Open Patent No. H5-29777, and performs signal receiving and delivery for an industrial control system. The function of this kind of input-output unit are to receive signals from and to deliver signals to the detector terminals of flowmeters, pressure gauges, and the like, and the manipulating terminals of electric valves, and the like, and to connect them to higher control devices. In particular there are input-output units used for analog signals which use 4mA to 20 mA, etc., analog instrumentation signals, signals from thermocouples, and the like. Also, there are input-output units used for digital signals which handle ON and OFF signals, pulse string signals, and the like. A specific configuration is, for example, disclosed in Japan Laid Open UM No. H4-88093.
In the past, the number of signals that can be handled by one unit has been small and the input-output cards that are accommodated in the slots of an input-output unit have been relatively large. This is because of the low integration density of the ICs which comprise the electric circuits that perform signal conversion. As a result, the number of signal wires connected to one unit has also been small, and it has been easy to secure space for positioning terminal blocks. However, as the integration density of the ICs becomes higher the number of signals handled by one unit also increases, e.g., to 16 points, 32 points, 64 points, etc. But, when the number of input-output signals is simply increased, because a large number of signal wires have to be handled in a confined space, the wiring operation is time consuming and there is the danger of the wires being mistakenly connected to the wrong terminals.
To maintain certain input-output cards, it is necessary to remove them from the input-output unit, and the interruption of control in the control loop that accompanies the maintenance may adversely affect the system being controlled. For this reason, in the case of an input-output unit for control use, it is desirable that the control loops of each of the signals be handled by an independent input-output card, and that even when a certain card is being maintained, the other cards remain loaded in the unit and continue to control the loops. Thus, in input-output units for control use, a separate card is usually provided for each control loop. However, because the input-output card shape and dimensions are becoming smaller than they have been in the past, there arises the problem that when conventional loading and removal mechanisms are used, the number of loading and removal members becomes large and the loading and removal operations become awkward.
In some cases, an input-output unit performs relatively simple intermediary functions, such as display, warning signal receipt, and control. However, in the past, input-output units of different types have been used to handle analog signals and digital signals. In such cases, for the manufacturer, there is the problem that the number of components required increases and the managing of the parts becomes complex. Manufacturing thus becomes high number of parts, low volume type of manufacturing; hence, the benefits of mass-production are lost and costs become high. From the user's point of view, there is the problem that because the shapes and dimensions of the various input-output units are not necessarily standardized, when the units are to be accommodated in a container, the combinations of input-output units that can be installed together are limited, and the freedom with which the units can be selected, is limited.
When the outside signal wires are connected to an input-output unit of this kind, a terminal block is used. For example, an ON and OFF signal is assigned a terminal set comprising a plus terminal and a minus terminal. In the case of a thermocouple input, a circuit for performing cold junction compensation is provided. When a large number of signals are to be inputted, common minus terminals are sometimes used. However, there is the problem that when a different terminal block is designated for each purpose, the number of molds for the plastic molding increases and the initial investment in manufacturing equipment, increases.
When connectors are used for the connections to an input-output unit of this kind, an interface section equipped with a display is used. However, when display devices are mounted in the interface section, there is the problem that when light guides are used, the number of components increases and the assembly operation becomes complex.